1. Field of the Invention
The present invention relates to a projection TV set apparatus, more particularly to a projection lens assembly of a projection TV set apparatus which can prevent a reduction in focusing performance of projection lenses caused by a rise in temperature during operation.
2. Description of the Related Art
Rear projection type projection television set apparatuses have larger screens than regular television sets and enable large-sized displays. In such rear projection type projection TV set apparatuses, projection cathode-ray tubes (CRT) produce red (R), green (G), and blue (B) images which they project to a screen. The viewer sees the picture on the opposite side of the screen from the side where the images are projected.
In front of the projection CRTs, lens assemblies are provided which focus the images produced from the projection CRTs to form the picture on the screen. Each lens assembly consists of a mirror sleeve and at least one lens fitted inside the mirror sleeve. In general, there are a combination of plastic lenses and glass lenses in the mirror sleeve of the lens assembly. The mirror sleeve is typically comprised of a metal or plastic.
The coefficients of thermal expansion of plastics, metals, and glass fall into the following relationship: Ep&gt;Em&gt;Eg, where Ep is the coefficient of plastics, Em is the coefficient of metals, and Eg is the coefficient of glass.
The projection CRTs become high in temperature during operation and therefore usually are cooled at the front.
The fronts of the projection CRTs can become as high as 90.degree. C. in temperature, however. Accordingly, the lens assemblies also become heated and problems arise as a result of the differences in the coefficients of thermal expansion of the components of the assemblies.
For example, if the mirror sleeve is made of a metal, a plastic lens will be subjected to pressure from the mirror sleeve due to its having a larger coefficient of thermal expansion than the mirror sleeve. The lens face will deform in shape and the focusing performance will therefore drop. A glass lens, further, has a smaller coefficient of thermal expansion than the mirror sleeve and therefore a gap will be caused at the fitting portion between the mirror sleeve and the glass lens. As a result, the plastic lens and glass lens will become off-centered from each other. In this case as well, the focusing performance will drop.
Note that even if the mirror sleeve is made of plastic, a similar problem will arise with the glass lens.